Question: In a manufacturing facility, and especially with a VFD (which I understand likes cleaner air), how important is it to vent the intake and exhaust of the compressor to outside? I can’t see air up on the roof being much cooler than inside the plant in the hot summer and in the winter you would mostly have to draw in plant air anyway due to the temperature.
Answer: It depends on the environment, but the most important thing is venting the hot air away from the compressor (not necessarily outdoors). Often, with no ducting to separate hot and cold air, or leaky ducting, the hot air discharged by the compressor will go right back into the intake, in a short circuit, causing the compressor to overheat.
It is important that the ventilation duct not represent a restriction to flow, so must be well sized and not subject to negative pressures due to other plant air exchangers. Best case is to vent the heat outdoors in summer and to cooler areas of the plant in winter.
Care needs to be taken with ventilation air inlet ducting — it can’t fall below freezing at any point, including when the compressor is off, or freeze-up can occur. The compressor air to air cooler is a place where water condenses, so freezing can wreck the cooler and shut down (or prevent the start) of the compressor.
In dirty environments, finding a clean supply of cooling air is wise, but be aware that often the VFD cooling intake is separate from the main cooling air intake. In the case of dirty environments some manufacturers have the option of remotely installing the VFD in a clean electrical room.
You have a point about rooftop air, it is often hot, but likely cleaner. But location of the intake source is sometimes important. One industrial plant placed the intake near their evaporative water-cooling system. The chemicals from the water mist caused contaminant problems in the compressor lubricant.
Best practices systems in locations with summer/winter conditions is to have both intake and outlet connected to outdoors with mixing damper between the ducts to condition the cold air in winter, keeping it at a minimum temperature of about 50° F. A secondary automatic damper would direct heat to the plant in winter and outdoors in summer.
In tropical environments that are never below freezing, the compressors are usually placed outdoors in a rain protected location, but again care must be taken to ensure the hot air discharge in not ingested by the compressor intake. This air should not be directed at a neighboring compressor or air dryer.